Domestically raised poultry, such as chickens, turkeys, ducks, geese, guineas, pheasants, and quail, are subject to a variety of diseases and infections after hatching. Some resistance to disease is provided by naturally-occurring antibodies and virus-neutralizing gamma globulins in the yolk of the egg, which is carried by the chick immediately beneath the skin of the abdomen. The yolk contents are absorbed into the digestive tract of the chick over a seven to nine day period after hatching. See, e.g., C. R. Parkhurst and G. J. Mountney (1989), Chapter 5, "Incubation and Hatchery Management," in Poultry Meat and Egg Production, Van Nostrand (New York), the disclosure of which is incorporated herein by reference.
Supplementary medications can be administered to poultry by several methods, including subcutaneous injection and eye drops. Subcutaneous injections commonly are performed in the necks of newly hatched chicks on an assembly line basis, and equipment for this purpose is available commercially. In this procedure, the chicks are manually picked up one by one and their necks are placed against an automatic injection device; an injection needle is quickly advanced into the chick's neck, a measured dose of medication is injected, and the needle is withdrawn. The medication injected in this manner diffuses rapidly into the chick's vascular system.
In an effort to provide poultry with a measure of immunity or resistance to disease upon hatching, medication can also be administered before hatching. Generally, eggs to be treated are placed on end with the air sac at the top; a small hole is formed through the shell at the top, and an injection needle is passed downwardly through the hole, and desirably into the amnion, into which the medication is discharged. Sometimes the embryo itself is unintentionally injected and may die as a result.
If the medication is a soluble vaccine, unintentional injection of the vaccine into the air sac can be effective, however cell-associated vaccines are typically ineffective if injected into the air sac. Egg injection methods and devices are described in Sharma et al., U.S. Pat. No. 4,458,630, Christensen, U.S. Pat. No. 4,604,968, and Hebrank, U.S. Pat. Nos. 4,681,063 and 4,903,635. As described above, injection of medication into the amnion makes the entire quantity of the medication immediately available to the embryo.
Of particular concern to the poultry industry is the disease known generally as coccidiosis, caused by protozoal parasitic organisms of the genus Eimeria. See, generally, "Coccidiosis", pp. 153-157, in Avian Disease Manual, C. E. Whiteman and A. A. Bickford, eds., Kendall/Hunt Publishing Co., 1989, the disclosure of which is incorporated herein by reference. Active and passive immunizations of adult poultry against this disease have been successfully performed on commercial scales for many years. However, only limited success has been achieved in broiler chickens. The reason is that broilers routinely reach market by 6 weeks of age. Using conventional methods of commercial-scale immunization, this is simply not a sufficient time period for the bird's immune system to develop protective immunity.
A procedure termed "trickle vaccination" has been used as a possible route by which effective immunity can be achieved in juvenile poultry. This procedure, as provided in the "Cocci-Vac" product available from Sterwin, Inc., requires that 200 oocysts (a developmental stage in the life-cycle of the Eimeria parasite) be administered per os to each chick within the first 2 days after hatching. When this number of oocysts is ingested during the early neonatal period, the chick typically will immediately develop protective immunity. While from a theoretical viewpoint this method of vaccinating juvenile poultry against coccidiosis may have merit, from a practical standpoint there has not, to date, been a feasible commercial-scale method demonstrated to insure that each chick ingests the required 200 oocysts. See, e.g., P. L. Long et al., Exp. Parasitol. 16:1-7 (1965), N. N. Sharma, J. Parasitol., 50:509-517 (1964), and M. E. Rose et al., Parasitol., 102:317-324 (1990).